This command migrates a repository from passphrase mode (removed in Borg 1.0) to
repokey mode.

You will be first asked for the repository passphrase (to open it
in passphrase mode). This is the same passphrase as you used to use for this
repo before 1.0.

It will then derive the different secrets from this
passphrase.

Then you will be asked for a new passphrase (twice, for safety).
This passphrase will be used to protect the repokey (which contains these
same secrets in encrypted form). You may use the same passphrase as you used
to use, but you may also use a different one.

After migrating to repokey mode, you can change the passphrase at
any time. But please note: the secrets will always stay the same and they
could always be derived from your (old) passphrase-mode passphrase.